(a) Field of the Invention
The present invention relates in general to preventing reverse current flow in switching power supplies. More particularly, it relates to an improved control circuit and method for preventing reverse current flow in synchronous rectifiers used in switching power supplies.
(b) Description of Related Art
Power supplies are used in many industrial and aerospace applications, and also in consumer products. Some of the requirements of power supplies are small size, light weight, low cost, and high power conversion efficiency. Some power supplies also require electrical isolation between the source and the load, low harmonic distortion for the input and output waveforms, and high power factor if the source is ac voltage.
DC power can be generated directly from a dc source, or converted from an ac source. If an ac source is used, then an ac-to-dc converter must be provided. In these converters, electrical isolation can only be provided by relatively bulky line frequency transformers. The ac source can be rectified (i.e., converted to dc) to generate uncontrolled dc, then a dc-to-dc converter can be used to convert the uncontrolled dc to a controlled dc output. Electrical isolation between the input source and the output source in the dc-to-dc converter is achieved by using a high-frequency (HF) transformer. HF transformers have small size, light weight, and low cost compared to bulky line frequency transformers.
Switching power supplies use power semiconductor switches that can be commanded to ON and OFF switching states to control the power into their associated power conversion elements, resulting in high efficiency, small size, and light weight. With the availability of fast switching devices, HF magnetics and capacitors, and high-speed control ICs, switching power supplies have become very popular. A particularly advantageous switching power supply uses pulse width-modulated (PWM) converters to convert an alternating voltage of variable amplitude and frequency to a variable dc voltage. These converters employ square-wave pulse width modulation to achieve voltage regulation. The average output voltage is varied by varying the duty cycle of the power semiconductor switch. The voltage waveform across the switch and at the output are square wave in nature, and they generally result in higher switching losses when the switching frequency is increased. Also the switching stresses are high with the generation of large electromagnetic interference (EMI), which is difficult to filter. However, these converters are easy to control, well understood, and have wide load control range.
Diode rectifiers are often used as the main rectifying (or power-conversion) element of a switching power supply. So-called "synchronous" rectifiers, most commonly silicon MOSFETs, are often used in switching power supplies to achieve greater power conversion efficiencies than are available from diode rectifiers. In general, synchronous rectifiers can be commanded, via control circuitry, to conduct significant current in both directions, while diodes are only capable of conducting significant current in one direction.
Typically, the control circuitry for a synchronous rectifier switches the rectifier ON when the power supply main switch is switched OFF, and the rectifier control circuitry employs various methods to prevent both the rectifier and the power supply main switch from being ON at the same time. However, in some applications, for example, where the power supply inductor is operating in a discontinuous conduction mode or where multiple power conversion modules are wired in parallel for redundancy and/or current sharing, the synchronous rectifier remaining ON when the output current of the converter reverses direction causes undesirable reverse current flows. There is, accordingly, a need for a rectifier control circuit and method that prevents undesirable reverse current flows in synchronous rectifiers.